This invention relates generally to internal combustion engines that propel motor vehicles and have both a variable valve actuation mechanism and a NOx adsorber catalyst for adsorbing Nox (oxides of nitrogen) in engine exhaust. A more particular aspect of the invention relates to a control strategy for regenerating the catalyst through use of the variable valve actuation mechanism.
Oxides of nitrogen have been identified as contributors to smog. A NOx adsorber catalyst placed in the exhaust system of an internal combustion engine can adsorb substantial amounts of oxides of nitrogen from engine exhaust passing through it so that they do not escape to the ambient atmosphere. A NOx adsorber catalyst may offer an advantage over those catalysts that are intended to be used with internal combustion engines running mainly at stoichiometric because it can allow an engine to run on a relatively leaner fuel-air mixture that renders the engine somewhat more efficient than when running at stoichiometric or richer.
As the engine operates, the NOx adsorber catalyst adsorbs more and more oxides of nitrogen and eventually must be regenerated. Generally speaking, a NOx adsorber catalyst adsorbs NOx in engine exhaust when the engine is running relatively leaner. The catalyst is regenerated by temporarily enriching the fuel-air mixture, resulting in release of the adsorbed oxides of nitrogen as gaseous nitrogen and water through catalytic aided chemical processes.
Known methods for enriching a fuel-air mixture for regeneration of a NOx adsorber catalyst include the use of intake throttling and external exhaust gas recirculation (EGR) in conjunction with control of engine fueling. The process may also be controlled by the use of feedback from a suitable exhaust sensor.
It is believed that the present invention can provide more efficient regeneration of a NOx adsorber catalyst and/or the capability for regenerating the catalyst essentially with inclusion of a suitable operating program in the engine control and without additional hardware.
When an engine has a variable valve actuation mechanism under control of a processor-based engine control, the timing of the opening and closing of cylinders valves, i.e. intake and/or exhaust valves, can be varied. The invention arises from a recognition that the variable valve actuation mechanism, in conjunction with proper fuel injection management, can be used to transition the fuel-air ratio between lean and rich without an external throttle in the intake system of the engine. Proper transitioning of the fuel-air ratio can efficiently regenerate a NOx adsorber catalyst in the engine exhaust system.
By varying the timing of intake valve operation through the use of a variable valve actuation mechanism, the amount of charge air that is captured in an engine cylinder and subsequently compressed can be controlled. Control of fueling in conjunction with control of intake valve timing will control fuel-air ratio, and with both fueling and valve timing being under control of a processor-based engine control, fuel-air ratio can be effectively controlled in a manner that is both fast and accurate.
As a piston is downstroking in an engine cylinder during an intake stroke, the corresponding intake valve is typically open. As the piston reaches bottom dead center (BDC) and begins a compression upstroke, the volume of the cylinder begins to decrease. If the intake valve remains open, some of the charge air will be forced through it back into the intake system. The time at which the intake valve closes therefore determines the amount of air that will remain in the cylinder for compression. Setting the amount of fuel to be injected for that amount of trapped air therefore establishes the fuel-air ratio of the mixture that will be combusted when the mixture ignites under the force of compression by the piston as the compression upstroke concludes. At each operating condition, there is an optimum intake valve closing for fuel economy and air-fuel ratio. Retarding intake valve closing pushes air in a cylinder back into the intake system, and advancing intake valve closing shortens the intake stroke. Therefore, retarding intake valve closing or advancing intake valve closing during the engine cycle result in a lesser amount of air in the combusted mixture, and hence a richer mixture for a given amount of fuel.
If the engine has a turbocharger, the regeneration process should take that fact into account and may temporarily alter turbocharger operation in a manner that enables the desired regeneration to be accomplished with effectiveness and efficiency.
Principles of the invention can be embodied in an engine control as part of an overall engine control strategy. The creation of an engine and an operating strategy that can regenerate a NOx adsorber catalyst in conjunction with an overall control strategy, is seen to be a desirable objective because it can make the regeneration process transparent to an operator of the vehicle, requiring no special measures on his or her part.
The present invention attains the desired objective through the control of the variable actuation mechanism for the engine cylinder valves by an engine control system embodying solid-state, processor-based electronics. The processor can process certain data inputs to develop data outputs defining valve opening and closing times that are most appropriate to the inventive strategy. The data outputs are converted into electric signals that are delivered via electric circuits to actuators of the variable valve actuation mechanism to open and close the intake valves at proper times for accomplishing regeneration.
Because certain principles of the present invention include changing the time in the engine operating cycle when the intake valves for the engine cylinders open and close, the variable valve actuation mechanism of the engine must be effective on each such cylinder valve. It may also be effective on exhaust valves. An example of such a mechanism comprises an electric actuator for opening and closing each corresponding cylinder valve in accordance with the electric signal applied to the actuator. This type of engine is sometimes referred to as a camless engine.
The invention is believed to provide an advantageous solution for regeneration because it is possible to implement in an existing engine at no added hardware cost by providing the processor with an operating program embodying an appropriate algorithm, provided that the engine, has a processor-based control and a variable valve actuation mechanism for at least the cylinder intake valves. When the variable valve actuation mechanism also controls timing of the cylinder exhaust valves, they may be timed in relation to intake valve timing to provide internal exhaust gas recirculation (iEGR) for use as a factor in the regeneration strategy. Procedures for initiating and terminating regeneration are also disclosed.
Accordingly, a fundamental aspect of the present invention relates to a novel strategy for regeneration of a NOx adsorber catalyst in an exhaust system of an internal combustion engine, especially a compression ignition, or diesel, engine that also has a variable valve actuation mechanism for varying the timing of at least the cylinder intake valves.
One general aspect of the claimed invention relates to a method of regenerating a NOx adsorber catalyst that adsorbs NOx in exhaust passing through an exhaust system of an internal combustion engine that has a variable valve actuation mechanism for varying the opening and closing times of intake valves that control flow into cylinders of the engine from an intake system. The method comprises, with the engine running under its own power, operating the variable valve actuation mechanism to change the timing of operation of the intake valves so as to transition the fuel-air ratio of mixture that is being combusted in the cylinders from a relatively leaner mixture to a mixture that is sufficiently richer than stoichiometric to effectively regenerate the catalyst.
Another general aspect relates to an engine that embodies a control for performing the foregoing method.
still another general aspect relates to a control for controlling a variable valve actuation mechanism of an internal combustion engine to regenerate a NOx adsorber catalyst in an exhaust system of the engine while the engine is running under its own power. The control causes the variable valve actuation mechanism to change the timing of operation of the intake valves so as to transition the fuel-air ratio of mixture that is being combusted in the cylinders from a relatively leaner mixture to a mixture that is sufficiently richer than stoichiometric to effectively regenerate the catalyst.
The foregoing, along with further aspects, features, and advantages of the invention, will be seen in this disclosure of a presently preferred embodiment of the invention depicting the best mode contemplated at this time for carrying out the invention. This specification includes drawings, briefly described below, and contains a detailed description that will make reference to those drawings.